When you set up collections and do not select a picture to represent a collection, a placeholder box will display on your website next to your collection on the Collections overview page. 

To correct this, select an image to represent the collection

• login to FASO account Control Panel 
• click Artwork icon 
• click Add/Edit Collections 
• click Edit to the right of a collection 
• scroll down
• for CollectionPicture, click Choose an Image to find your image visually, or click on the drop-down to select by title
• click Save Changes

If you have not assigned any images to the collection you will get this message:
You have not uploaded any images yet.

From this FAQ:
How do I choose or change the image that represents my collection?